Baffle type muffler with sound absorbing material



Oct.h14, 1952 D. J.,w. cuLLuM BAFFLE TYPE MU'T'F'LER WITH SOUNDABSORBING MATERIAL Filed Dec.'

I NVENTOR ,lllli l l l Irllllllvlxll j DOUGLAS J.

W ULLUM BY ATTORNEY Patented Oct. 14, 1952 ABSORBING MATERIAL DouglasJack Wayin Cullum, London, England Application December 19, 1949, SerialNo. 133,810

In Great Britain October 14, 1948 1 Claim. (C1. 181-50) In theconstruction and testing of jet propulsion engines,- such as in researchand test workshops, the engines are suitably supported on their benchesorequivalent with their jet nozzles directed towardsan aperture orrespective apertures in the wall of the workshop in order that theiiarnes and products of combustion of the jets shall be discharged intothe outside atmosphere. A great noise is caused by the jets during test,and it is known to employ a so-called detuner arrangedoutside thebuilding with its mouth located in or passing through the wall apertureto receive the issuing jet and partially absorb and minimise the sound.The aperture is sometimes provided with a pair of doors which are openedto allow the detuner to be run up so as to dispose its mouth inproximity 'to the engine during use. These detuners are usually in theform of a perforated cylindrical tube concentrically mounted withinanother tube to form an annular space which is lagged withsound-absorbing material.

`The present invention is an improvement in noise-suppressing apparatusfor the foregoing purpose, and has for its objects to provide optimumoperating conditions for the engine while suppressing the noise, and toabsorb the sound of the issuing jet to a greater degree than is attainedby known detuners.

According to the present invention there is provided noise suppressingapparatus for use in testing jet engines comprising a detuner tubehaving an interior hollow pocket-like baille, projecting so as to be inthe path of the gas stream and having its mouth directed towards theentrance to the tube. In practical embodiments of the invention the saidbaille is a hollow perforated or equivalent structure of metal or othersuitable material supported substantially concentrically within thedetuner tube and arranged so that there is an annular space between itswall and the inner wall of the tube at all points between the mouth andrear end of the baille. Preferably a part or the whole of the baille, inlongitudinal section, is progressively smaller towards its rear end. Toform a complete lobstruction across the detuner tube the edge of themouth of the baille, such as a ilange, is iixed in contact with theinner wall of the detuner so that its body is spaced from the inner wallof the detuner tube. A part or the whole of the detuner and, where aninner sound absorbing core is used, as hereinafter described, at leastthe section of the detuner tube which embraces this core, is constructedwith an outer jacket which is lagged interorly, e. g. by rock t 2 wool,to provide sound absorption, and the lagging is protected by aperforated metal liner tube.

In the use of the improved device the flames and combustion productsfrom the jet exhaust are directed into the interior of this pocket-likebaille and escapethrough the perorations into the space between thebaiile and the surrounding detuner wall. The baille may be of conical,pyramidal or semi-spherical form, or may be cylindrical of smallerdiameter than the inner Wall of the jacket and with an out-turned flangeat the mouth for attachment to the jacket, orit may be anysuitable'combnationof these shapes, e. g. cylindrical with a sphericalor conical end. Instead of a single pocket it may be an assemblyofpockets in laminated array, slightly spaced to give passage forthegases, either with or without sound absorbing material interposedbetween the plates.

One baiiie is preferably disposed at or near the exit end of thedetuner, and may be associated with a core supported within the detunerto lie longitudinally along the centre thereof, said core preferablyhaving pointed deflector ends, one of which terminates within thesaidbaille. Said inner core is a hollow member containing a suitablesound-absorbng packing and the wall of the body thereof is perforated.The flames and combustion products transverse the space between the coreand wall of the detuner, eventually entering the baille and passingalong the space between the end of the core and the wall of the baille.

The accompanying drawing illustrates diagrammatically an embodimentincluding the various features of the invention.

In this drawing l is the jet engine exhaust which is located opposite anaperture in a Wall 2, through which aperture is projected the leadingend 3 of the detuner tube while this leading end may take any suitableform. It may be telescopically mounted in a main part 3a which leads toan entrance 3b of frusto-conical interior. The detuner part 3b extendsinto a second tubular part 4 which consists of a jacket having aninterior sound absorbing lining which is protected by a perforated liner5. Near the rear or exit end of the detuner part 4 the interior passageis obstructed by conical baille plate 6 which is perforated and whichhas a mouth flange secured to an annular shoulder 1 formed on the innerwall of the detuner partit. The baffle mouth is co-extensive with theperforated liner 5, with the apex of the cone directed towards the rear.The baffle, in conjunction with the rear internal Wall 8 of the detuner,forms a diverging annular passageway 9 through which the gases pass toatmosphere.

Increased sound absorption is provided by the core I0 which is a hollowperforated metal cylinder of circular cross section coaxial with thedetuner part d and lled with rock wool or other suitable sound absorbingmaterial. The core is of smaller diameter than the inner wall of thedetuner tube so as to provide an annular gas channel II leading to thebaille 6, the ends I2, I3 of the core being conical deflectors whichguide the gases in the required direction, for which purpose they areadvantageously plain and not perforated, the rear end I3 being locatedwithin the baffle 6 as shown. The pointed end of the baille B may beblanked off by a plate I4 which may or may not be perforated. The tip ofthe cone in rear of the plate I4 may be imperforate. I

Although any suitable cross section may be given to the detuner 4, thecore IU and the baille Ii, it is advantageous to adopt a circularsection for all three, giving the core a torpedo shape with conical endsas shown, and the baille a conical shape, but the angle of the cone ofthe baille need not necessarily correspond with the angle of the pointof the core end I3; in fact as shown it is preferable that the angle ofthe baille is more acute than the angle of the core point whereby theannular space l5 between core and baffle gradually increases in crosssection towards the outlet.

For utmost erhciency, the area of the annular space 9 outside the bailleand between the baille and detuner wall taken at any plane normal to theaxis of the detuner should be not less than (and preferably is greaterthan) the superficial area in the aggregate of all the perforations inthe baffle between said lplane and the base of the baille, this arearelationship being particularly applicable when a conical baille isemployed as shown.

I claim:

Noise suppressing apparatus for use in testing jet engines, comprising acylindrical muffler tube, an interior lining of sound absorbing materialfor said tube, a protective perforated liner for said material, saidtube with its sound absorbing lining and perforated liner comprising agas entrance portion of frusto-conical section providing greaterthickness of sound absorbing material at the entrance end of saidportion than at the remote end of said portion, an intermediate portionwherein the annular space for the sound absorbing material between thetube and the liner is of uniform cross-section along the entire lengthof the intermediate portion, and a gas discharge portion wherein theliner flares outwardly towards the outer end of the tube providing adiminishing thickness of sound absorbing material between the tube andthe liner, a hollow conical perforated baille located within said gasdischarge portion with the mouth of the baille directed towards theentrance portion of the tube and co-extensive with the outer end of theintermediate portion of the tube, an imperforate conical cap attached tothe rear end of the frustoconical baille, an imperforate plate closingthe outer end of said baille, a perforated hollow core supportedco-axially within the intermediate portion of the tube and extendingfrom inside the entrance end of the intermediate portion to inside themouth of said baille and forming with the intermediate portion anannular gas channel leading to the mouth of the baille, imperforateconical deflectors closing the ends of the core, and a filling of soundabsorbing material within the core.

DOUGLAS JACK WAYTH CULLUM REFERENCES CITED The following references areof record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,067,200 Shlosberg July 8, 19131,709,333 Webb Apr. 16, 1929 1,811,762 Schnell June 23, 1931 1,847,830Hills Mar. 1, 1932 2,017,744 Bourne Oct. 15, 1935 2,373,231 Demuth Apr.10, 1945 2,514,996 Faust July 11, 1950 2,523,260 Campbell Sept. 26, 1950

